1. Field of the Invention
The present invention provides a resin bonded rare earth magnet with superior corrosion resistance due to suppression of roughness on the surface of the magnet.
2. Description of the Prior Art
Rare earth magnets made from rare earth metals and transition metals have found widespread use in recent years due to the remarkable superiority of their magnetic characteristics compared to ferrite or Alnico magnets. However, because Nd—Fe—B rare earth magnets easily oxidize under the condition of high temperature and high humidity, resulting in the formation of rust and degradation of magnetic characteristics, resin or metallic coatings have been applied to the magnet surface to inhibit oxidation. Especially for bonded magnets made from Nd—Fe—B quenched powder and organic resin, it has been customary to use spraying or electrolytic deposition to apply a resin coating or nickel plating to the surface of the molded body.
In rare earth bonded magnets, which are mainly used in PM stepping motors or the spindle motors for CD-ROM drives, high quality is required for magnetic characteristics and corrosion resistance in order to achieve more compact size and lower power consumption. Especially, as the storage density of hard disk drives increases, this is accompanied by requirements for cleaner and faster spindle motors, and accordingly it has become important that the magnets mounted in such motors have sufficient corrosion resistance and strength, and are free from adhering organic particles, etc.
Conventionally, the surface of bonded magnets is coated with epoxy or acrylic resin to improve corrosion resistance. However, because magnetic particles of irregular sizes ranging from a few tens to a few hundreds of microns in diameter are fixed in a small quantity of resin bond, there will be a large number of protrusions or depressions in the surface of the magnet. Because of this, the thickness of the coating will not be even and the smoothness of the surface cannot be ensured. Defects such as pinholes and air bubbles easily occur, which will lead to deterioration of the corrosion resistance. Moreover, since the coating may not fully cover some projecting magnetic particles which are exposed at the magnet surface, such particles may break off during the rotation of a spindle motor and cause interference with the rotation.
To overcome this problem, attempts have been made to use magnetic particles having a particle size of 10 microns or less to minimize roughness on the magnet surface. However in this case the magnetic density produced by compression molding is insufficient and the desired magnetic characteristics cannot be obtained. It is also impossible to greatly reduce the roughness on the surface of the magnet by improving the methods of forming the coating using coating or electrolytic deposition. Accordingly, a thick film coating with a thickness of several tens of microns must be employed in order to achieve sufficient corrosion resistance. However, if such a magnet coated by a thick film is used in a spindle motor, because of the thickness of the coating, the effective magnetic flux per volume of the magnet will be reduced, which is inconvenient for the motor characteristics. In these circumstances, there is a requirement for a powerful resin bonded rare earth magnet with good corrosion resistance, in which the roughness of the magnet surface has been suppressed.
In view of the conditions described above, the present invention has the objective of providing a resin bonded rare earth magnet, compression molded from rare earth-transition metal powder and thermosetting resin, in which a corrosion inhibiting coating made from synthetic resin which covers the surface of the compression molded magnet is made as thin as possible while preventing the occurrence of defects such as air bubbles and pinholes.